Several clinical and experimental observations suggest that factors which influence erythropoiesis also affect platelets and hemostasis. Thrombocytopenia and altered coagulation are often associated with erythrocytosis secondary to hypoxemia associated with cyanotic congenital heart disease, chronic respiratory insufficiency and exposure to high altitude or hypobaric pressure. The etiology of these abnormalities remains unclear as some reports have presented evidence for consumption coagulopathy while others have suggested a decreased production of hemostatic components. Anemia, on the other hand, is often accompanied by thrombocytosis. An increased incidence of thromboembolic phenomena has been reported in patients with persistent anemia after splenectomy. To elucidate the interactions between erythropoiesis and hemostasis we propose to study the response of platelets and coagulation factors to various hematocrit levels and factors which influence the hematocrit, such as oxygen tension. The sequential response of hemostatic factors to various levels and durations of hypobaric hypoxia, erythrocyte hypertransfusion and anemia will be studied. The effects of the spleen on hemostasis will also be studied in rats with various hematocrit levels since splenectomy seems to increase the incidence of thromboembolic phenomena in patients whose anemia persists. Platelet number, production rate, survival and function as well as the plasma coagulation system will be evaluated in the various experimental conditons. These studies should provide an understanding of the roles erythropoiesis and hypoxia play in the development of abnormal hemostasis in clinical states of erythrocytosis and anemia.